The present invention relates to a compressor. In particular, the invention relates to the inlet arrangement of a centrifugal compressor and to a turbocharger incorporating such a compressor.
A centrifugal compressor comprises an impeller wheel, carrying a plurality of blades (or vanes) mounted on a shaft for rotation within a compressor housing. Rotation of the impeller wheel causes gas (e.g. air) to be drawn into the impeller wheel and delivered to an outlet chamber or passage. In the case of a centrifugal compressor the outlet passage is in the form of a scroll volute defined by the compressor housing around the impeller wheel and in the case of an axial compressor the gas is discharged axially.
The turbocharger is a well-known device for supplying air to the intake of an internal combustion engine at pressures above atmospheric (boost pressures) and is widely used on automobiles and the like. The compressor of a turbocharger is driven by an exhaust gas turbine that is mounted on a common shaft. Exhaust gas from the internal combustion engine flows through the turbine and drives the turbine wheel in rotation, which, in turn, rotates the compressor impeller. Air is drawn through an axial inlet of the compressor housing and compressed air is delivered to the intake manifold of the internal combustion engine, thereby increasing engine power.
One aspect of turbocharger control is to ensure stable operation by avoiding what is known as surge. If the turbocharger is operating at a relatively low compressor volumetric air flow rate and a high boost pressure the air flow into the compressor may stall and the operation of the compressor is interrupted. Following stall, the air flow tends to reverse through the compressor until a stable pressure ratio is reached at which the air can flow in the correct direction. This process repeats and results in pulsations in the air flow known as surging. Maximum operating efficiency of the engine is achieved by operating close to the surge limit and a surge margin is built into the control process to ensure that the turbocharger operates at a safe distance from the surge condition.
In some turbochargers the compressor inlet has a structure that has become known as a “map width enhanced” (MWE) structure. An MWE structure is described for instance in U.S. Pat. No. 4,743,161. The inlet of such an MWE compressor comprises two coaxial tubular inlet sections, an outer inlet section or wall forming the compressor intake and inner inlet section or wall defining the compressor inducer, or main inlet. The inner inlet section is shorter than the outer inlet section and has an inner surface that is an extension of a surface of an inner wall of the compressor housing which is swept by edges of the impeller wheel blades. The arrangement is such that an annular flow path is defined between the two tubular inlet sections, the path being open at its upstream end and provided with apertures or a slot (hereinafter referred to as the “MWE slot”) at its downstream end that communicate with the inner surface of the compressor housing that faces the impeller wheel. In operation, the MWE slot allows additional air to be drawn into the compressor under high flow (near choke) conditions, however its most important function is at lower flow rates and, in particular, as the compressor approaches surge. Under these conditions the MWE slot allows the flow to reverse (which is now the prevalent flow regime in parts of the compressor) and to be re-circulated to the intake, thus delaying surge.
It is well known that the MWE structure stabilises the performance of the compressor increasing the maximum flow capacity and improving the surge margin, i.e. decreasing the flow at which the compressor surges, so that the range of engine r.p.m. over which the compressor can operate in a stable manner is increased. A given compressor can thus be matched to engines with a wider speed range. This is known as increasing the width of the compressor “map”, which is a plot of the compressor characteristic.
It has been shown that increasing the length of the inner inlet section and therefore the annular flow path improves the surge margin, as described in our European patent No. 1473465. However, the benefits can drop with increasing length particularly since the efficiency of such a compressor can be reduced.
It is also known in some compressor embodiments for the surge margin to be improved by allowing the MWE slot to be open to the surrounding engine environment. This can be achieved, for example by removing the outer inlet wall. An open MWE slot has safety and operational implications. In particular, hot air is discharged through the slot near surge and needs to be channel to an area where it is not a safety hazard. Moreover, it is possible that debris can be drawn into the compressor with air via the MWE slot when it is operating at high flow. In some circumstances it is possible that fragments of the vanes break off from the impeller wheel and are blown out of the MWE slot with obvious safety risks.